How to Calculate Return Air Duct Size [3 Simple Steps]

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Your air conditioning bill might be high because it’s drawing too much air from outside. This might be due to an improperly sized return air duct. So, to deal with it you need to calculate the proper return air duct size.

How to calculate return air duct size?

It’ll depend on what type of duct you use. If you’re using a rectangular duct the math is simple. Multiply the size of your system by 144 sq. inch and then sq.root the value. You’ll get the required measurement. For a circular duct, calculate the CFM and then compare it to a chart for duct size.

In short, that is how you get the duct size. Keep reading though. I’m going to break down the process in more detail. 

How Do You Calculate Return Air Duct Size: 3 Step Guide

Calculating the return air duct size is simple. I’ve described the entire process in a few steps for your convenience. Just follow along. 

Step 1: Gathering Necessary Information

The size of the return air duct will depend on your air conditioning system’s size. Check the label on the condenser. You’ll see a model number. 

There’ll be a two-digit even number in the model number. Take this number and multiply by 1,000. Then divide the answer by 12,000. The answer you get is the tonnage or size of your system. 

Let’s say there’s a 24 in the model number. Then the tonnage is (24,000/12,000) or 2 tons. 

If you’re putting in a central air conditioning unit you’ll be incurring quite an extensive cost. You’ll need to understand the costs associated with central AC.

Step 2: Picking Your Duct Shape

Depending on what shape your air duct is, your calculation will defer. Therefore, you’ll first need to decide on the shape of the duct. It can either be rectangular or circular. 

You’ll need to consider the space available in the walls, installation costs, and air pressure. If you’re using a low-pressure system you’re better off with a rectangular duct. Most residential systems are low pressure. 

The rectangular ducts will cost more to install. However, they’ll seal better and create a tighter system. Circular ducts on the other hand will create a more efficient flow of air. 

They are also easier to install. However, they do require heavier sealing and more space. On top of that, you’ll need to insulate your ductwork. That’ll take even more space.  

Choose a duct shape based on these considerations. 

If you do choose a circular duct you’ll need a good sealant. Here are a few I recommend:

Product 1 
Product 2

Either of these will work like a charm and keep your ducts from leaking any air. 

Step 3: Calculating the Duct Size

Once you’ve picked the duct shape, you can finally get to the size calculation. The calculations aren’t difficult. I’ll break down the formulae so you can easily understand the process. 

Size of Rectangular Duct

If you’re using a rectangular duct, multiply the system size by 144 sq. inch. That’ll give you the total area of the duct. You can either square root the number to get the dimensions of a square duct. Or you can decide on a height × width measurement. 

For Example: You have a 2-ton system. The duct size will be 2 144 = 16.97 sq. inches. You can round this up to 17 inches. However, since 17 × 17 is not a standard size, you can use an 18 × 18 duct.  

Size of Circular Duct

For a circular duct, you’ll first need the CFM rating of your system. If you don’t have the actual rating use 400 CFM per ton. Follow the duct size chart to find the diameter of the round duct. 

Continuing with my 2-ton system example, airflow = (2 × 400) or, 800 CFM. Now comparing it to the chart I get a diameter of 16 inches.  

You’ll see tables for both metal ducts and flex ducts on charts for circular ducts. The lists are generally for supply air ductwork. However, even for return air flex duct is an option.   

Why Do You Need Return Air Ducts?

The return air duct (RAD) moves air from the room. It takes the air inside the room and feeds it into your air-conditioning system. Basically, the purpose of the return air duct is to circulate the air in your room.

Now, why is that done?

Cost Considerations

Your air conditioner spends energy to heat up or cool down the air. The greater the difference between the air temperature and the desired temperature the greater the cost. That’s because the system will need to use up more power to change the temperature.

The RAD lowers cost by circulating air that is already near the desired temperature. Since previously heated or cooled air is used, less energy is expended. As a result, you save up on power bills. 

Air Circulation 

Return Air Ducts work by circulating air in your rooms. This is very important to ensure proper air pressure inside the house. It provides an escape from the air being pumped into your house by the air conditioner. 

The supply and return air ducts together ensure proper circulation. Supply air duct brings in conditioned air and the return duct removes the already present air. Thus creating a proper circular flow. 

Air Purification

Circulated air picks up dust and debris inside your home. This debris can damage your air conditioner. The vent for the return air duct comes with a filter that helps remove these particles. This ensures the quality of air in your rooms. 

That concludes my thoughts on air return ducts. 

FAQs

Question: Is going under the required size okay?

Answer: No. If you lower your duct size, your airflow will be restricted. There will be greater resistance within the duct. This resistance will result in higher noise. It will also cause greater power consumption. 

Question: Can a duct be too large?

Answer: Yes. There is such a thing as a duct that is too large. A duct significantly larger than the optimal size will cause loss of pressure, and inefficient performance. 

Question: How often should I replace my air filters?

Answer: It depends. In a house with one person changing the filter once a year is enough. But when there are more people and pets, it should be changed once every 2 months.

Conclusion

I’ve provided the answer about how to calculate the return air duct size. Everything you need to consider and even the formulae you’ll need is all here. 

All you gotta do is follow along. Just remember, it is okay to go a little above the recommended size not under.

Well, I hope I’ve helped you figure out what size return air duct you need.

Goodbye!

Richard Allen